Disable duplicate notification creation by default, and introduce 'allow_duplicate' param for override

Generally, it's undesirable to have duplicate notifications for a given
user-item-secondary_item-component-action combination (as when, say, someone
mentions you many times in a single message thread). This changeset introduces
duplicate protection, which can be overridden by passing the 'allow_duplicate'
param to bp_notifications_add_notification().

Fixes #5297

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8100 cdf35c40-ae34-48e0-9cc9-0c9da1808c22